U.S. Pat. No. 4,528,848 discloses such a metering apparatus, where a material stream is conveyed over a measuring path while charging a load measuring device. The conveyor is in the form of a rotor having an essentially vertical axis and pockets moved in a circular orbit over the measuring path. A housing surrounds the rotor and includes a charging aperture and a discharging aperture which are rotationally displaced from each other. A pneumatic conveying system has feed lines, respectively communicating with an air inlet to said housing and the discharging aperture. The pneumatic conveying system is an open system, i.e. pressurized air from a pressurized air source is applied to the apparatus and the air conveying the metered pourable material is directly guided to a device of use of the pourable material which may be a burner of a stove. Though a large through-put is achieved with such an apparatus at relatively high accuracy, temperature changes of the air or variations in humidity thereof may affect the measurement. Furthermore, expelled residual air will contain small quantities of material polluting the atmosphere.
In the book "Dosieren in der Kunststofftechnik", Editor: Verein Dt. Ingenieure, VDI-Ges. Kunststofftechnik, 1st Edition 1978, pages 70/71, systems for continuous gravimetric mixing and pneumatic conveying of a number of components have been disclosed using differential metering scales operating with worm conveyors for supplying gravimetrically metered pourable material through a mixing device to a collecting worm at the outlet of which there is provided a mixing device. The system is extremely complex, specifically if it is necessary to process large quantities. A specific problem is the conveying of the various components to the collecting worm.
German Laid Open Patent Application No. 35 20 657 discloses a multiple system using a single weighing device for three material containers. The containers are emptied by pressurized air one after the other. There is no closed loop air path.
German Laid Open Patent Application No. 26 06 941 discloses a gravimetric metering apparatus. Pourable material is supplied from a storage bin through a cellular sluice into a pneumatic feeding duct for being conveyed through a cyclone separator and a worm into a scale container.
German Patent No. 12 86 958 discloses an apparatus for intermittently conveying of pourable materials using a container filled by applying vacuum and emptied by applying pressure.
German Patent No. 27 55 671 discloses a multiple station conveying system using a closed loop suction air path including a separating filter provided with means for cleaning thereof by applying pressurized air in opposite direction of the suction airstream intermittently.